Conventional image sensors have a limited dynamic range, such as from 60 dB to 70 dB. Many scenes, however, have a much larger dynamic range. For example, natural scenes often have a dynamic range of 90 dB or more. Therefore, techniques have been developed to extend image sensor dynamic range.
For example, multiple exposure techniques have been developed to obtain a single high dynamic range image from multiple exposures of a conventional image sensor. In these techniques, multiple image data sets are generated, where each image data set is generated with a different exposure time duration of the image sensor's photodiodes. The multiple image data sets can be sequentially generated one after another. Alternately, the multiple image data sets can be generated partially in parallel using a rolling shutter technique, thereby reducing delay between sequential exposures of a given pixel. The high dynamic range image is subsequently generated by selecting between the multiple image data sets according to one or more criteria, for each pixel location represented by the high dynamic range image. In multiple exposure techniques, the image data sets do not temporally overlap at a given pixel location, resulting in motion artifacts when the image scene changes between the multiple image data sets.
High dynamic range image sensors have also been developed. One example of high dynamic range image sensors is split-pixel image sensors. These image sensors are characterized by each pixel including at least two different photodiodes optimized for light of a different intensity range. For instance, FIG. 1 is a top plan view of a prior art split-pixel image sensor 100 including multiple pixels 102. Each pixel 102 includes four large photodiodes 104, 106, 108, 110 and four small photodiodes 112, 114, 116, 118. Large photodiodes 104, 106, 108, 110 are optimized for capturing high intensity light, and small photodiodes 112, 114, 116, 118 are optimized for capturing low intensity light. Each pixel 102 include a Bayer color filter array, such that large photodiodes 104 and small photodiodes 112 are adapted to capture red-color light, large photodiodes 106, 108 and small photodiodes 114, 116 are adapted to capture green-color light, and large photodiodes 110 and small photodiodes 118 are adapted to capture blue-color light.
Large photodiodes 104, 106, 108, 110 generate a first image data set optimized for high intensity light, and small photodiodes 112, 114, 116, 118 generate a second image data set optimized for low intensity light. A high dynamic range image is generated by selecting between the first and second image data sets according to one or more criteria, for each pixel location represented by the high dynamic range image. The first and second image data sets typically at least partially temporally overlap, and split-pixel image sensor 100 is therefore significantly less prone to motion artifacts than imaging systems using multiple exposure techniques. However, the fact that each pixel 102 includes at least eight photodiodes reduces available surface area for large photodiodes, thereby making it difficult to obtain high sensitivity under low light conditions.